Sydney's Opera House and Harbour Bridge plunged into darkness Saturday for the Earth Hour environmental campaign, among the first landmarks around the world to dim their lights for the event.

Lights will go out in some 7,000 cities and towns from New York to New Zealand for Earth Hour which this year aims to raise hundreds of thousands of dollars for green projects.

"The thing about Earth Hour is that is reminds people that it needs to be a global response," said Anna Rose, national Earth Hour manager for Australia, the country in which the event began in 2007.

"It's quite beautiful when people turn off their lights in Earth Hour to know that they are joining with people in 154 countries."

Australians were this year focusing on the Great Barrier Reef, the huge coral structure off Queensland which conservationists fear will be irreversibly damaged by climate change without urgent action.

The Earth Hour movement will see world landmarks including the Empire State Building, the Eiffel Tower and the Kremlin switch off their lights for 60 minutes at 8:30 pm local time on Saturday.

The WWF organised event is being coordinated from Singapore, with the stars of new movie "Amazing Spider-Man 2" set to help switch off lights on the city-state's skyline in the upmarket Marina Bay district.

Earth Hour began in 2007 in Sydney, but the idea quickly spread around the world and hundreds of millions of people are estimated to have turned their lights off for the event last year.

The event is a symbolic action rather than one to reduce carbon pollution, but it has drawn criticism, including from Danish political scientist Bjorn Lomborg who argues it does little for the real problem of global warming and diverts resources from other problems.

"This celebration of darkness sends the wrong message, Lomborg said in a statement this week.

"While more than a billion people across the globe make a symbol of foregoing non-essential electrical power for one hour a year, another 1.3 billion people across the developing world will continue to live without electricity as they do every other night of the year."

Rose said it was obvious that switching lights off for one hour would not solve the world's environmental problems, but she said the event had raised awareness about climate change in the community.

"It's important as a moment in which Australians can have this conversation about climate change," she said.